Introduction to Spectroscopy

Introduction to Spectroscopy • Spectroscopy involves an interaction between matter and light (electromagnetic radiation) • Light can be thought of as waves of energy or packets (particles) of energy called photons • Properties of light waves include wavelength and frequency

Electromagnetic Radiation Spectrum • There are many wavelengths of light that can not be observed with your eyes

Spectroscopy used in Chemistry • When light interacts with molecules, the effect depends on the wavelength of light used • This chapter focuses on IR spectroscopy

15.2 IR Spectroscopy • An IR spectrophotometer irradiates a sample with all frequencies of IR light • Detector determine the degrees of absorption at all frequencies to show the presence of certain bonds (functional groups) Media used to determine the IR spectra: • Samples are deposited neat on a salt (NaCl) plate. • Or, sample dissolved in a solvent or embedded in a KBr pellet

Bond vibration patterns • Molecular bonds can vibrate by stretching or by bending in a number of ways • This chapter will focus mostly on stretching frequencies

Energy level from Bond Vibration • The energy necessary to cause vibration depends on the type of bond

IR absorption from Bond Vibration • For each bond, vibrational energy levels are separated by gaps (quantized) • If a photon (light) strikes the molecule with the exact amount of energy needed, a molecular vibration will occur • Energy is eventually released from the molecule generally as heat • Infrared (IR) Light generally causes molecular vibration • HOW IR light is absorbed  molecular structure

IR Spectroscopy • A signal on the IR spectrum has three important characteristics: wavenumber, intensity, and shape

IR Frequency: Wavenumber

Classification of IR in Structure Analysis • The region above 1500 cm-1 : diagnostic region. • The region below 1500 cm-1 : the fingerprint region. FINGERPRINT REGION DIAGNOSTIC REGION

IR of –OH group: Ether vs. Alcohol CH3OCH3 CH3CH2OH

IR of C=O group: Alcohol vs. Carboxylic acid Ethanol (-OH) acetic acid (>C=O & -OH)

Wavenumber: Bond and Mass • The wavenumber for a stretching vibration depends on the bond strength and the mass of the atoms bonded together • Heavier atoms causes lower frequency IR

IR Wavenumber: Bond and Mass Stronger bond causes higher frequency stretch vibration (higher wavenumber) Heavier atoms causes lower frequency stretch vibration

IR wavenumber: Hybridization • Same C-H bond, different hybridization: • C-H bond (carbon being sp hybridized) is strongest.

IR pattern for C-H of dif. hybridization • The region ≈3000 cm-1 : sp3, sp2, sp

IR for hydrocarbon w/o unique C-H • List the absorption around wavenumbers (in cm-1 ) of 3000 cm-1 for the molecules below: A. 3300 B. 3100 C. 2900 I II III

Resonance affects wavenumber • Consider a carbonyl that has two resonance contributors • C=O ~1720 cm-1, C-O 1120 cm-1 • If there were more contributors with C-O single bond character than C=O double bond character, the observed wavenumber is in between C-O and C=O bonds.

Examples of Resonance Effect

15.4 IR Signal Strength • The strength of IR signals can vary

Molecular Symmetry vs. Signal Strength • Note the general strength of the C=O stretching signal vs. the C=C stretching signal • Typical C=C bond has absorption at 1500-2000 cm-1 • A symmetrical molecule with a completely nonpolar C=C bond: 2,3-dimethyl-2-butene, no IR signal in the 1500-2000 cm-1 region

#Bonds affects IR Signal Strength • Stronger signals are also observed when there are multiple bonds of the same type vibrating • Although C-H bonds are not very polar, they often give very strong signals, due to the large number of C-H bonds • Because sample concentration can affect signal strength, the Intoxilyzer 5000 can be used to determine blood alcohol levels be analyzing the strength of C-H bond stretching in blood samples

15.5 IR Signal Shape • Some IR signals are broad, while others are very narrow • O-H stretching signals are often quite broad

Hydrogen bonding affects Shape • When possible, O-H bonds form H-bonds that weaken the O-H bond strength • The H-bonds are transient, so the sample will contain molecules with varying O-H bond strengths • Varying O-H bond strength leads to broad band signals. • If no H-bonding: The O-H stretch signal will be narrow if a dilute solution of an alcohol is prepared in a solvent incapable of H-bonding

Example of signal for O-H bond • In a sample with an intermediate concentration, both narrow and broad signals are observed. • Explain the cm-1 readings for the two O-H stretching peaks

IR Signal Shape for Carboxylic Acid • Consider how broad the O-H stretch is for a carboxylic acid and how its wavenumber is around 3000 cm-1 rather than 3400 cm-1 for a typical O-H stretch

Carboxylic Acid forms Dimers • H-bonding is often more pronounced in carboxylic acids, because they can forms H-bonding dimers (two molecules bonded through H-bonding)

Practice: Predicting IR Signal Shape • For the molecule below, predict all of the stretching signals in the diagnostic region: • X-H: O-H, -C-H, =C-H • X=Y: C=C, C=O (conjugated with C=C)

Signal Shape for Amine (N-H) • Primary and secondary amines exhibit N-H stretching signals. • Tertiary amines has no N-H bond. • Because N-H bonds are capable of H-bonding, their stretching signals are often broadened • N-H bonds are less polar than O-H bond, thus weak signals.

IR Signal for Amines

IR Signal Shape for –NH2 • The appearance of two N-H signals for the primary amine is NOT simply the result of each N-H bond giving a different signal • Instead, the two N-H bonds vibrate together in two different ways

Symmetry of Vibration • A single molecule can only vibrate symmetrically or asymmetrically at any given moment, so why do we see both signals at the same time? • Similarly, CH2and CH3 groups can also vibrate as a group giving rise to multiple signals • Practice with conceptual checkpoint 15.10

Effect of solvation on IR • Explain how an experiment involving isotopic labeling might be used to explore the type of fragmentation that occurs in the MS analysis of organic compounds.

15.6 Analyzing an IR Spectrum • Table 15.2: key signals for functional groups • Often, the molecular structure can be identified from an IR spectra • Focus on the diagnostic region (above 1500 cm-1) • 1600-1850 cm-1 – check for double bonds (C=O or C=C) • 2100-2300 cm-1 – check for triple bonds (CN or CC) • 2700-4000 cm-1 – check for X-H bonds (C-H, O-H, N-H) • Analyze wavenumber, intensity, and shape for each signal

Analyzing an IR Spectrum • Focus on the 2700-4000 cm-1 (X-H) region • Practice with SkillBuilder 15.1

15.7 Using IR to Distinguish Between Molecules • As we have learned in previous chapters, organic chemists often carry out reactions to convert one functional group into another • IR spectroscopy can often be used to determine the success of such reactions • For the reaction below, how might IR spectroscopy be used to analyze the reaction? • Practice with SkillBuilder 15.2

Practice: Using IR to Distinguish Between Molecules • For the reactions below, identify the key functional groups, and describe how IR data could be used to verify the formation of product • Is IR analysis qualitative or quantitative?

Additional Practice Problems • Explain how an experiment involving isotopic labeling might be used to explore the type of fragmentation that occurs in the MS analysis of organic compounds.

Introduction to Spectroscopy